Mechanisms for harnessing and utilizing the sun's energy have taken many forms and one such mechanism has come to be known as a flat plate collector. In such a device, a planar structure is provided with a surface which is absorptive to the solar energy and which converts that energy to heat, and then fluid is circulated over or in conductive contact with that surface to absorb the heat from the surface area whereupon the fluid is conducted away carrying the energy with it. The energy contained in the heated fluid has many end uses as a heat source, the end uses not being the subject of this application.
The most common features of flat plate collectors heretofore known include water flowing through copper tubing in an insulated enclosure. The surfaces of the copper tubing are provided with energy absorptive coatings, or other energy absorptive surfaces are provided in contact with the copper tubing so as to absorb the solar energy converting it to heat and conducting the heat to the water. A glass or other transparent cover is provided minimizing convection losses and, at the same time, creating a "greehouse effect" thereby reducing reradiation losses from the system.
It is known that the wavelength of energy radiated from a heated body is inversely proportional to the temperature of that body. The spectrum of solar energy reaching the earth's surface is comprised substantially of wavelengths in the range of approximately 0.3 to 3.0 microns. The energy radiated from an absorptive surface having a temperature of 40.degree. C is composed of wavelengths between 8.0 and 10.0 microns. Thus, it is apparent that by providing a material which is transparent or absorptive to the incoming radiation having the short wavelengths and opaque or absorptive to the energy radiated from the absorptive surface having longer wavelengths would greatly increase collector efficiency. Such materials are available and known generically as interference filters. Some glass and some plastics have these characteristics and are commonly used in flat plate collectors.
The principal shortcoming of prior flat plate collectors has been their complexity and use of expensive materials such that such devices have been unable to compete with fossil fuels in the energy market. Although inexpensive to operate, their high initial cost, including installation costs, has made them noncompetitive.